For many "downhole" operations, i.e., those operations carried out by means of tools lowered into boreholes, it is necessary to be able to determine the orientation of the tool when it is emplaced at the selected depth. Downhole tools are frequently lowered into the borehole on a single cable, or "wireline", and during the course of lowering the tool, may rotate unpredictably such that its orientation can no longer be determined with certainty from the surface.
The need to determine downhole tool orientation is particularly acute in the case of a "multiple zone completion", i.e., where an oil or gas well is completed so as to permit production from more than one production stratum or zone. Such multiple zone completions are often carried out by running two or more strings of production casing in a side-by-side relationship into a single well bore which penetrates all the zones of interest. Depending upon the design of the particular well, a completion may be a "cased" completion wherein the strings of production casing are themselves contained within a larger diameter casing installed in the borehole, or a completion may be an "open hole" completion wherein the production casing is cemented directly into an uncased well bore. Especially in open hole completions, it is important that the multiple strings of production casing be positioned away from the sides of the borehole to allow the ready circulation of cement between the casing and borehole walls. This allows good cement adhesion and thickness so that a satisfactory seal can be achieved at each production zone. Production casing is generally positioned by the use of so-called "centralizer" devices. A typical centralizer comprises a pair of longitudinally spaced-apart retaining clamps which are attached around the exterior of the casing string and connected to one another by a set of longitudinally oriented "bow spring" straps which bow outward between the retaining clamps and thus serve to hold the casing string away from the walls of the borehole.
In order to produce from a particular zone, a tool known as a perforator is lowered into one of the strings of casing and positioned at the depth of the zone to be produced. The perforation is provided with explosive charges or guns which fire jets or bullets through the wall of the casing string and into the formation to be produced. The perforator typically used in a multiple zone completion is of the type that fires its jets or bullets in a single direction. These jets or bullets must be directed so that the other casing string or strings of the multiple zone completion will not be perforated or otherwise damaged. In this manner, each zone to be produced is perforated from a selected casing string, so that it is possible to produce each zone independently of the others. It will be appreciated that in order to do this with certainty and safety, the orientation of the perforating tool with respect to the other casing strings must be known just prior to firing.
Representative prior art solutions to the problem outlined above are U.S. Pat. No. 3,704,749 to Estes et al.; U.S. Pat. No. 3,776,323 to Spidell et al.; and U.S. Pat. No. 3,964,533 to Basham et al.
The Estes et al. patent describes a device for orienting a tool such as a perforator with respect to a ferrous body such as an adjacent casing string under the general conditions already outlined, wherein the orienting device utilizes an exciter coil producing an alternating electromagnetic field and a pair of receiver coils longitudinally spaced from the exciter coils, the disposition of the receiver coils being such that the voltages induced therein vary differentially with the angle presented by the detected ferrous body by reason of the distortion of the otherwise axially symmetric field.
The Basham et al. patent describes another orienting device in which motion is imparted to a permanent magnet assembly to generate a moving magnetic field and receiver means are provided such that measurable signals are induced therein when the magnetic field is distorted due to the presence of a ferrous anomaly. The receiver means is rotated to produce an azimuthal scan such that there are induced in the receiver means signals from which the azimuthal location of the anomaly can be determined.
The Spidell et al. patent describes yet another orienting device which comprises a source producing a narrow, laterally directed beam of radiation and a laterally directionally-sensitive radiation detector unit, adapted to receive radiation resulting from scattering of the source beam radiation in the adjacent environment. Means are provided for the rotation of the direction finder device about its longitudinal axis so that an annular portion of the surrounding medium would be scanned by the source and the detector to locate the adjacent tubing strings.
While prior art orientation devices such as those described by Estes et al. allow the correct orientation of the perforator tool with respect to the adjacent tubing string in most cases, experience with such devices has indicated that in some circumstances, for example in the proximity of large ferrous masses such as casing collars, the prior art orientation devices experience a "weak signal" failure mode wherein the overall signal produced by the orientation device becomes weak and orientation is uncertain. Alerted by the weak signal, however, a trained orientation device operator will recognize the "weak signal" failure mode and will not fire the perforator and risk damaging the adjacent tubing strings.
Of greater concern to many orientation tool operators than the readily recognizable "weak signal" failure mode is a second, less common, failure mode of prior art orientation devices, which for the purposes of this disclosure will be termed the "signal reversal" failure mode. Although the "signal reversal" failure mode occurs in only a small fraction (under 1%) of orientation jobs, it is a persistent problem which has eluded solution for over ten years. In the "signal reversal" failure mode, a seemingly strong and clear signal is received from the orientation tool which, in fact, is reversed up to 180.degree. from the actual orientation. Because of the strong signal, the "signal reversal" failure mode is not readily recognizable to a trained orientation device operator and typically results in the device operator orienting the perforator such that it fires toward, instead of away from, adjacent tubing strings, thus damaging or destroying the adjacent strings.
While the incidence of such "signal reversal" failure mode is small, it is economically significant to the users of orientation devices, such as oil field wireline services, since when a failure occurs and tubing strings are damaged, a typical wireline service must pay for repairs to the well and such repair costs may exceed the gross profit on the job by ten times or more. Thus, while the "signal reversal" failure mode occurs only occasionally, it is economically significant. A need exists, therefore, for an orientation device that allows the correct orientation of a directionally-acting tool, such as a perforator, in a borehole having multiple casing strings with respect to adjacent casing strings, and further which is resistant to the "signal reversal" failure mode. As previously discussed, the need for such a device has existed in industry for over ten years, but such need was not met until development of the current invention.